1. Field of the Invention
This invention relates to a phosphazene material and a method for producing the material, and more specifically, this invention relates to a polyester incorporating a trimeric phosphazene material and a method for making the phosphazene-based polyester material.
2. Background of the Invention
Polyphosphazene polymers are a versatile class of hybrid organic-inorganic materials that have many remarkable properties. Soluble and hydrolytically stable phosphazene polymers have a variety of uses, including as membranes for the removal of water from aqueous solutions, for organic separations, and as solid polymer electrolytes. Additionally, they may be formed as water-soluble high polymers, or as non-flammable fluids.
The key to these phosphazene-based polymers is their solubility in certain solvents that allows for facile formation into useful materials such as thin dense films. The variety of applications that have been proposed for phosphazene polymers is a clear reflection of diversity of the chemistry that they possess.
The backbone of a phosphazene polymer consists of alternating phosphorus and nitrogen atoms with alternating double and single bonds. Phosphorus, in this configuration is pentavalent with two substituents on phosphorus. Initially, phosphazene linkages are formed from the condensation of phosphorus pentachloride and ammonium chloride that yields a colorless crystalline solid that is easily sublimed. Heating of this material yields an elastomer with an empirical formula of PNCl2 that was originally termed xe2x80x9cinorganic rubberxe2x80x9d. However, this rubber is hydrolytically unstable and evolves hydrochloric acid upon exposure to water. The polymer can be deliberately modified with organic nucleophiles via the labile chlorines. This strategy yields a variety of robust polymers and is still in widespread use.
Polymerization of phosphazene material potentially can be performed to yield three distinct backbone structures: linear, cyclolinear, and cyclomatrix, see FIG. 1. R and Rxe2x80x2 represent monodentate pendant groups and bidentate crosslinking groups, respectively. Of these structures, the linear configuration has been the most extensively studied. Cyclolinear structures have been the least studied due to the complexity of synthesis.
Cyclomatrix polyphosphazenes are more common than the cyclolinear structures as there are several synthetically accessible routes. Interest in cyclomatrix phospha-zenes is driven by the desire to reduce material cost while taking advantage of the inherent stability of the inorganic phosphazene backbone. They are potentially more useful materials than cyclolinear materials. For example, crosslinked hexa-[4-hydroxy-phenoxy]cyclotriphosphazene (FIG. 1) was found to react readily with hexamethylenetetraamine at 200xc2x0 C. to yield a durable but intractable solid structural material. M. L. Stone, Mat Res. Soc. Symp., 305, 85 (1993).
Uncrosslinked hexa-[4-hydroxyphenoxy]cyclotriphosphazene has been synthesized through a two-step protection-deprotection methodology. A. Medici, G. Fantin, P. Pedrini, M. Gleria, F. Minto Macromolecules, 25(10), 2569 (1992). This method entailed the synthesis of hexa-[4-methoxyphenoxy]cyclotriphosphazene and hexa-[4-phenoxyphenoxy]cyclotriphosphazene from hexachlorocyclotriphosphazene and the corresponding substituted phenol. Unmasking of the hydroxyl functionality was performed by hydrolysis of the corresponding alkyl or aromatic ether to yield an uncrosslinked trimeric building block.
An alternate one-step synthesis exists to produce the trimeric phosphazene material, wherein hexachlorocyclotriphosphazene is reacted with hydroquinone in the presence of pyridine. R. G. Rice, B. H. Geib, and L. A. Kaplan, U.S. Pat. No. 3,121,704 (1964). D. A Femec and R. R. McCaffrey, J. Appl. Poly. Sci., 52, 501 (1994). As the reaction proceeds, the pyridine acts as a base for proton abstraction from the hydroquinone to produce the active nucleophile that displaces the labile chlorine on the phosphazene ring. However, characterization of this reaction via 31P NMR spectroscopy revealed several peaks, suggesting inhomogeneous phosphorus speciation. This result is attributed to reaction of both hydroxyl sites on the hydroquinone resulting in a partially cross linked structure. Prevention of crosslinking by using an excess of hydroquinone was unsuccessful. Also, the requirement of hydroquinone is regrettable inasmuch as that chemical is carcino- genic.
U.S. Pat. No. 4,533,726 awarded to Myers et al on Aug. 6, 1985 discloses a polymeric reaction product incorporating phosphazene. However, that polymer exhibits hydrophilic properties and is not ideal for homogenization with nonpolar solvents.
U.S. Pat. Nos. 3,994,996 and 4,111,701 awarded to Franko-Filipasic on Nov. 30, 1976 and Sep. 5, 1978 respectively, discloses a liquid phase phosphazene-containing polymer. The polymer is combined with fibers to render the later fire resistant. The phosphazene units are linked together by an oxygen bridge connecting phosphorous atoms of neighboring units.
U.S. Pat. No. 3,867,186 awarded to Hook et al on Feb. 18, 1975 also discloses using phosphazene material as a flame retardant.
U.S. Pat. No. 4,321,217 awarded to Allcock et al on Mar. 23, 1982 discloses a method for attaching halogen directly to phosphorous on phosphazene via a metal intermediate. This method is designed to improve thermal and hydrolytic properties of phosphazene.
A need exists in the art for a phosphazene-based polyester and a method to produce the polyester. The polyester must be soluble in typical industrial solvents so as to facilitate the polyester""s incorporation into membranes and other useful configurations. The material should not automatically polymerize during formation, nor should the method utilize known carcinogenic reactants.
An object of the present invention is to provide a phosphazene material and a method of forming phosphazene material that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a method for producing a macromolecule comprising cyclomatrix phosphazene subunits. A feature of the invention is that the phosphazene subunits are trimeric in nature. An advantage of the invention is that the trimeric nature of the molecule facilitates polymerization of the molecule only when the molecule is subjected to deliberate polymerization processes.
It is another object of the present invention to provide a method for using cyclomatrix phosphazene as a subunit in phosphazene-containing polyester molecules. A feature of the invention is the trimeric structure of the cyclomatrix phosphazene comprising a core, an aromatic region surrounding the core and an aliphatic moiety directed away from the core. An advantage of the trimeric structure is that additions to the structure are regio-specific and generally juxtaposed to the aliphatic moiety. Another advantage is that the aliphatic moiety donates electrons to the aromatic region, causing the region to become nucleophilic and therefore more reactive to the phosphazene core during formulation, resulting in higher yields and purer product.
Yet another object of the present invention is to form a polyester containing cyclomatrix phosphazene. A feature of the polyester is the existence of an aliphatic moiety attached to an aromatic moiety comprising the cyclomatrix phosphazene molecule. An advantage of the polyester is that it is fully soluble in common polar organic solvents such as acetone and tetrahydrofuran and therefore can be used as thin films in applications such as membranes.
Another object of the present invention is to provide a safer process for forming hexa [hydroxyphenoxy] phospbazene. A feature of the invention is the replacement of hydroquinone in typical cyclomatrix phosphazene production processes with a ten-fold excess of tert-butylhydroquinone. An advantage of this is the elimination of a suspected carcinogen from the production process.
A further object of the present invention is to form new cyclomatrix phosphazene-containing polyester polymers. A feature of the polymers is that they are formed from trimeric building blocks and varying length spacers between the building blocks. An advantage of the invention is that different spacer lengths confer different morphology to the resulting polyester.
Yet another object of the present invention is to provide new cyclomatrix phosphazene-containing polyester polymers. A feature of the invention is the incorporation of bidentate acid chlorides to provide ester couplings between trimeric phosphazene subunits, and organic spacers between phosphazene moieties. An advantage of the invention is that the utilization of the couplings and organic spacers accelerates the condensation reactions required for forming the polyesters. High yields result.
Still another object of the present invention is the utilization of phosphazene trimeric subunits to produce polyester macromolecules. A feature of the invention is that each subunit provides steric hindrance to prevent automatic oligomerization of the subunits. An advantage of the invention is that the subunit can be utilized in deliberate polymerization processes.
Briefly, the invention provides for a polyester comprising phosphazene molecules, an aromatic molecule attached to the phosphazene molecules and containing an alkyl moiety directed away from the molecules and an ester linking the phosphazene molecules to each other.
The invention also provides a method for synthesizing polyesters containing phosphazene comprising reacting hexa-(alkylhydroquinone)cyclotriphosphazene in a nonaqueous solvent with a bifunctional acyl halide. Generally, the bifunctional acyl halide is present in a molar ratio to the phosphazene of between 1.5:1 and 100:1.